https://codehaus-plexus.github.io/plexus-utils/ is rendered with the old https://maven.apache.org/skins/maven-default-skin/ instead of Fluido. Seems that the parent site descriptor was not correctly retrieved which sets the skin (https://repo1.maven.org/maven2/org/codehaus/plexus/plexus/18/plexus-18-site.xml).